The present invention relates to laminated safety glass which is commonly employed for windshields of an automobile and for other industrial uses. While most commercial present day automobile windshields comprise two sheets of glass and a thermoplastic interlayer, the present invention relates to laminated windows having an exposed layer of plastic resin, particularly "bilayer windshields" consisting essentially of one transparent glass sheet and one transparent sheet of plastic resin, such as polyvinyl butyral resin. The term "bilayer windshields" used throughout this specification is not limited to two layer laminates, but relates to any laminated window including a sheet of glass laminated to a layer of a plastic having a smooth surface (opposite that facing the glass) in pressurized engagement with a laminating mold during lamination. A thin protective coating such as polyvinyl Duoride may be present on the smooth surface. Such "bilayer windshields" may find use as side lights or back lights as well as windshields of automobiles or as closures for other vehicles as well as buildings. The term is also to be interpreted as covering laminated articles in which the glass sheet surface facing away from said plastic resin is in turn laminated to one or more plies of transparent material as in bullet-resisting glass and laminated windows having at least two glass plies alternating with plastic with an anti-lacerative coating of plastic on the glass surface facing the interior of the vehicle, and the like. U.S. Pat. No. 2,047,253 to Brooks suggests that a piece of laminated glass composition of two sheets of glass secured together by a single sheet of plastic material is less resistant to breakage by impact than is a single piece composed of a single layer of glass and a single layer of plastic each the same as where two pieces of glass are used. Falling ball tests show that laminates made with a single layer of glass is at least 50% stronger than laminates having two layers of glass, according to this patent. This improved strengthening of a so-called bilayer windshield is alleged to be due to the fact that there is no shearing effect when only one piece of glass is used while in the standard laminated glass windshield, the plastic tends to be sheared between the fractured layers of glass. Despite this suggestion in the Brooks patent, laminated windshields for automobiles have continued to be fabricated with two sheets of glass and a plastic interlayer. It is suggested that the reason for this lack of change in windshield structure has been due to the lack of a suitable fabrication technique for making bilayer windshields.
U.S. Pat. No. 2,120,628 to Reid claims a bilayer windshield having one glass sheet and one layer of a polyvinyl acetal resin treated in such a manner to produce properties superior for laminates. However, all the Examples receited in this patent refer to producing glass-plastic laminates in which the polyvinyl acetal resin sheet is interposed between a pair of glass sheets. Again, it is suggested that the absence of an Example disclosing how to fabricate a bilayer is due to the absence of knowledge of a suitable fabrication technique.
U.S. Pat. No. 2,184,876 to Sherts suggests laminating a single sheet of plasticized polyvinyl acetal to glass using an outer face of unplasticized resin to protect what whould be an exposed surface of plasticized polyvinyl acetal. A thin flexible sheet plated with chromium is suggested as a parting material over the resin sheet. This patent shows only a flat laminate whose shape can be accommodated by a thin chromium plated flexible sheet. Such a sheet cannot be shaped readily to conform to a concave shape and cannot be formed in a permnent shape except for a flat shape.
Any material used as a parting material or as a mold release agent between the mold and the plastic layer that is even slightly adherent to the plastic layer causes an uneven surface of the plastic sheet when the mold and the removed porting material are from the plastic layer after subjecting an assembly to heat and pressure to laminate the bilayer windshield. Any resulting optical defect annoys the driver and other occupants of the vehicle and is likely to be rejected commercially.
Curved bilayer windshields have an outer ply of glass having a generally convex surface facing the outside of the vehicle to which an inner plastic sheet coextensive throughout the entire extent of the outer glass sheet is laminated. The plastic inner sheet has a generally concave surface facing the interior of the vehicle. The problem of removing a mold from the plastic sheet of the bilayer formed by heat and pressure lamination is further complicated by the fact that the mold surface held in pressurized engagement against the concave surface of the plastic layer of the bilayer windshield so laminated must be convexly curved.
The resulting laminated bilayer windshield thus tends to wrap itself about the mold, making separating the mold from the bilayer windshield further complicated. The resulting transparent laminated window must have a relatively soft plastic inner sheet presenting a smooth, concave surface facing the interior of the vehicle that is free of optical defects laminated to at least one glass sheet that is harder that the inner plastic sheet. The relatively soft, plastic inner layer causes the resulting laminate to exhibit a lower Severity Index and less lacerative damage than a more conventional laminated glass windshield having an innermost glass ply.